Calcium sulfate having the chemical formula CaSO4.xH2O can be obtained from natural rock mines or by chemical synthesis. Calcium sulfate is traditionally used as a sulfur-based fertilizer, filler in paper, flux agent for casting-aluminum renal flux driers and pigment extender.
Water of crystallization is water that occurs in crystals but is not covalently bonded to a host molecule or ion. On the basis of the amount of crystal water, calcium sulfate can be classified as calcium sulfate dihydrate (CaSO4.2H2O, i.e., gypsum), calcium sulfate hemihydrate (CaSO4.0.5H2O, i.e., plaster of Paris) and calcium sulfate anhydrite (CaSO4). The chemical processes of hydration and dehydration are shown in FIG. 1.
The dehydration of the calcium sulfate dihydrate begins at approximately 90° C. during heating. After the temperature rises to the range of 90 to 150° C., the hemihydrate form is produced. When the heating is continued, the crystal water of hemihydrate will be further removed and calcium sulfate anhydrite is formed. The following are the chemical reaction formulae of the above reactions:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O  (1)CaSO4.½H2O+heat→CaSO4+½H2O  (2)
Calcium sulfate hemihydrate exists in two chemically identical forms: alpha calcium sulfate hemihydrate (alpha (α)-CSH) and beta calcium sulfate hemihydrate (beta (β)-CSH). Although they are chemically identical, the CSH is considered a specialty product due to the different characteristics of the two forms. In particular, the alpha form is a larger, regularly shaped crystal and has a low water demand for hydration reaction. About 25˜40 g of water are required to set a gypsum block from 100 g of the hemihydrate, and accordingly, the hydrated gypsum produced therefrom is a dense, high strength product. In contrast, the beta form is a smaller, irregular-shaped crystal and has a relatively high water demand; about 60˜80 g water are required to set a gypsum block from 100 g of the hemihydrate, and a lower strength hydrated gypsum product is produced. The α-calcium sulfate hemihydrate has a small exothermic peak after the endothermic peak; however, the β-calcium sulfate hemihydrate does not have this peak.
U.S. Pat. No. 5,614,206 adds water in different ratios to α-calcium sulfate hemihydrate and β-calcium sulfate hemihydrate to solidify them into samples with 1.2 cm diameter and 4 cm height, and performs degradation testing in water. The pure β-calcium sulfate hemihydrate is dissolved within 28 days, whereas 16.1% of the α-calcium sulfate hemihydrate is dissolved in 35 days. The results show that the more α-calcium sulfate hemihydrate there is, the slower the dissolution in water will be. Therefore, degradation of α-calcium sulfate hemihydrate is better suited for bone regeneration period.
U.S. Pat. No. 3,410,655 discloses a process for preparing α-calcium sulfate hemihydrate from gypsum by-product comprising elutriating calcium sulfate dihydrate with water to remove organic impurities at a temperature between 60° C. to 90° C., forming an aqueous suspension of calcium sulfate dihydrate having a pH between 1.5 and 6, adding a crystal form improving agent to the dihydrate suspension, and heating it in an autoclave at a temperature between 105° C. and 125° C. U.S. Pat. No. 4,091,080 provides a method for the production of α-calcium sulfate hemihydrate, comprising suspending hydrated calcium sulfate containing ½ to 2 moles of crystal water in an aqueous solution containing at least 10% by weight of magnesium salts or zinc salts of carboxylic acids and subsequently heating the resultant suspension under atmospheric pressure at a temperature in the range of 80° C. to the boiling point of said aqueous solution. U.S. Pat. No. 4,120,737 uses the waste effluent containing sodium chloride/calcium chloride ammonia from the ammonia soda process to manufacture sodium carbonate as material to produce α-calcium sulfate hemihydrate wherein 40 to 98% by weight of sulfuric acid is added and the reaction is carried out at a temperature between 95° C. and 100° C. under atmospheric pressure or a temperature between 140° C. and 160° C. under 5-10 atmospheres absolute.
U.S. Pat. No. 4,432,954 discloses that α-calcium sulfate hemihydrate is obtained by calcining calcium sulfate dihydrate with a particle size of 250 μm, generated with flue-gas desulfurization in power plants or by a chemical process, e.g., in the production of phosphoric acid. The calcining process comprises producing initially a dispersion from calcium sulfate dihydrate and diluted 15 to 55 wt % of sulfuric acid solution and heating the same at a predetermined rate to about 373 K with industrial, dust-free waste heat, whereupon the solid components are separated from the aqueous sulfuric acid and are dried. U.S. Pat. No. 4,842,842 discloses a method for producing alpha-form gypsum hemihydrate, comprising heating an aqueous slurry containing gypsum dihydrate and a salt of sulfosuccinic acid as the catalyst for crystallization, thereby converting gypsum dihydrate into alpha-form gypsum hemihydrate. U.S. Pat. No. 5,015,450 provides a method for making calcium sulfate alpha-hemihydrate by recrystallization transformation or recrystallization of calcium sulfate dihydrate in the presence of saturated steam at a temperature of 110° C. to 180° C. A molded body is formed from the calcium sulfate dihydrate which has a plurality of pores having a pore volume amounting to 15 to 60% of the total volume of the entire molded body.
U.S. Pat. No. 5,248,487 relates to a process for the conversion of calcium sulfate dihydrate originating from flue gas desulfurization units into an alpha-hemihydrate, wherein a portion of the scrubbing liquor is concentrated to a calcium chloride content of about 20% to 40% by weight as the salt solution, magnesium chloride or potassium chloride are added to the salt solution to increase the concentration of the magnesium chloride or potassium chloride to about 2 to 8% by weight magnesium chloride and about 0.5 to 2% by weight potassium chloride. U.S. Pat. No. 6,652,825 provides a method of producing α-calcium sulfate hemihydrate with 90 to 95% purity, including the steps of exposing a mixture including a calcium sulfate form selected from the group consisting of calcium sulfate dihydrate, calcium sulfate beta-hemihydrate, water-soluble calcium sulfate anhydrite, and mixtures thereof, water, and a crystallization catalyst, to microwave radiation to produce calcium sulfate alpha-hemihydrate; and separating at least a portion of the calcium sulfate α-hemihydrate to provide a solid comprising calcium sulfate α-hemihydrate and a filtrate and/or remainder that can be recycled to provide at least one of a calcium sulfate form, water, a crystallization catalyst, a crystal habit modifier, and a surfactant for use in production of additional α-hemihydrate. Evans et al. reported in 1995 that using microwave to remove moisture was feasible [“A Study of the Dielectric Properties of Gypsum and their Relation to Microwave Drying Behavior”, Proceedings of Microwave and High Frequency Heating Conference, St Johns College, Cambridge (1995)]; however, if the temperature is higher than 120° C., too much energy is provided so calcium sulfate anhydrite will be produced during the transformation of calcium sulfate hemihydrate.
U.S. Pat. No. 6,780,391 provides a method of producing α-calcium sulfate hemihydrate which is characterized by: forming an initial calcium sulfate dihydrate from potassium sulfate and calcium nitrate; drying the precipitate at 200° C. for 24 hours; subsequently rehydrating the calcium sulfate anhydrite in the form of a slurry by immersing it in deionized water; placing the resulting slurry at room temperature for three days; using 2.5 bar steam pressure to treat it for 2 hours and then drying the resulting products to obtain calcium sulfate alpha-hemihydrate. Obviously, the above process needs a longer production time and high pressure to perform the transformation of α-calcium sulfate hemihydrate.
In medical applications, calcium sulfate has been used as a bone graft substitute for regeneration or augmentation in many fields, from dentistry to orthopedics. For medical applications and particularly for implantation, high purity materials are obviously needed. Medical grade calcium sulfate (CS) is a biocompatible, bioabsorbable, and clinically versatile ceramic for use in bone repair. It is an osteoconductive material, so it can be used as a filler in regions with bone defects. Furthermore, antibodies, growth factors or demineralized bone matrix having osteoinduction material can be added to calcium sulfate so that calcium sulfate can be used as a carrier for bioactive materials to help the regeneration and healing of the regions with the bone defects.
According to ASTM F2224-03 Standard Specification for High Purity Calcium Sulfate Hemihydrate or Dihydrate for Surgical Implants, the total amount of heavy metals (such as mercury, arsenic, lead and cadmium) must be lower than 10 ppm and the amount of iron must be lower than 100 ppm. The α-calcium sulfate hemihydrate produced by using the by-product of calcium sulfate from the chemical processes in chemical factories may have residues of heavy metals and toxic substances. Therefore, there is still a need to develop a process for preparing α-calcium sulfate hemihydrate with high purity.